1. Field of the Invention
The present invention relates to a reflection-type display device and a method for producing the same.
2. Description of the Related Art
Liquid crystal display devices (hereinafter “LCDs”) of a reflection type, which perform display by utilizing ambient light as a light source, have been known. Since reflection-type LCDs do not require a backlight as do transmission-type LCDs, reflection-type LCDs are suitably used for various devices which must have a light weight and a thin thickness. In particular, reflection-type LCDs of an active matrix driving type, in which a switching element is provided corresponding to each pixel, are capable of performing display with a high resolution and high quality.
In a reflection-type LCD, ambient light which enters a liquid crystal layer is modulated in the liquid crystal layer, and thereafter reflected by a reflective layer so as to be utilized for displaying. Use of a retroreflection plate having retroreflection characteristics as such a reflective layer has been proposed (see, for example, Japanese Laid-Open Patent Publication Nos. 2003-195788 and 2002-107519, both of which have been filed by the Applicant). As used herein, a “retroreflection plate” is a device which reflects an incoming ray of light with a plurality of reflection surfaces, regardless of the orientation of the ray, in the direction in which the ray entered the device. For example, a retroreflection plate is composed of a two-dimensional array of minute unit features.
Reflection-type LCDs which employ a retroreflection plate as a reflective layer (“retroreflection-type LCDs”) do not require any polarizing plates, and hence there is no decrease in the efficiency of light utilization associated with the use of polarizing plates. Thus, retroreflection-type LCDs can perform brighter display. Moreover, retroreflection-type LCDs are considered as promising because of their potential ability to realize an improved display contrast ratio.
Hereinafter, the structure of a retroreflection-type LCD of an active matrix driving type will be described with reference to the accompanying drawings. FIG. 1A is a schematic cross-sectional view showing a retroreflection-type LCD. FIG. 1B is a plan view showing reflection electrodes in the display device of FIG. 1A. A structure as shown in FIGS. 1A and 1B is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-195788 filed by the Applicant.
As shown in FIG. 1A, the retroreflection-type LCD comprises: a front substrate 110 on which color filters 119, a transparent counter electrode 111, and an alignment film 112 are provided; a rear substrate 109 provided so as to oppose the front substrate 110; and a liquid crystal layer 113 interposed between the substrates 110 and 109. The rear substrate 109 includes: a TFT substrate 101 having a plurality of switching elements (TFTs), a retroreflective layer 106, and an alignment layer 118. The retroreflective layer 106 includes an insulating layer 102 which has a surface configuration that exhibits a retroreflection property; and a reflective metal layer 105 which is formed on the insulating layer 102 and presents an uneven surface corresponding to the surface configuration of the insulating layer 102. As shown in FIG. 1B, the reflective metal layer 105 consists of a plurality of reflection electrodes which are formed so as to be spaced apart from one another, corresponding to pixels (which define units of image displaying). Each reflection electrode is connected to a drain electrode 103 of a corresponding switching element on the TFT substrate 101, via a contact hole 104 which is formed in the insulating layer 102. The alignment layer 118, which is formed above the retroreflective layer 106, has protrusions and depressions corresponding to the surface configuration of the insulating layer 102. The liquid crystal layer 113 may be composed of, for example, a scattering type liquid crystal material which is capable of switching between a light transmitting state and a light scattering state (forward scattering) in accordance with a varying voltage which is applied between the counter electrode 111 and each reflection electrode 105. Although not shown in FIG. 1A, structures (called “spacers”) for controlling the interval between the substrates 110 and 109 are provided between the two substrates 110 and 109.
In a display device of this structure, the retroreflective layer 106 functions not only as a retroreflective layer but also as pixel electrodes. Hereinafter, the operation of this display device will be described.
While the liquid crystal layer 113 is controlled to be in a transmitting state, light from a light source which lies external to the display device or ambient light is transmitted through the front substrate 110 and the liquid crystal layer 113, and thereafter reflected by the retroreflective layer 106 in the direction in which the light has entered. From the display device under this condition, an image of the eye(s) of the viewer himself or herself is perceived by the viewer, whereby a “black” displaying state is obtained.
On the other hand, while the liquid crystal layer 113 is controlled to be in a scattering state, the light from a light source or ambient light which has been transmitted through the front substrate 110 is scattered in the liquid crystal layer 113. In the case where the liquid crystal layer 113 is a forward scattering-type liquid crystal layer, the scattered light is reflected by the retroreflective layer 106, further travels through the liquid crystal layer 113 (which is in a scattering state), and goes out in the viewing direction. Since the retroreflection property of the retroreflective layer 106 is counteracted by the scattering in the liquid crystal layer 113, the incident light does not go back in its incident direction. As a result, a “white” displaying state is obtained.
By performing display based on such operation principles, it is possible to realize white/black displaying states without employing polarizers. Therefore, there is no decrease in the efficiency of light utilization associated with the use of polarizers, and a reflection-type LCD having a display with a high brightness can be realized.
In any display device based on the operation principles as illustrated in FIGS. 1A and 1B, in order to further improve the contrast ratio during display, it is important to increase the retroreflectance of the retroreflective layer to reduce the amount of unnecessary light which reaches the viewer in a black displaying state.
As one type of retroreflective layer having a high retroreflectance, corner cube arrays are known. A corner cube array is a two-dimensional array of corner cubes (CCs) each composed of three faces which are perpendicular to one another. FIGS. 2A and 2B are a plan view and a perspective view, respectively, of a corner cube array. In Japanese Laid-Open Patent Publication Nos. 2003-066211 and 2003-185817, both filed by the Applicant, it is proposed to produce an array of minute corner cubes (micro-corner cube array, hereinafter abbreviated as “MCCA”), by anisotropically etching a substrate which has a crystal structure, and utilize the MCCA as a retroreflective layer of a reflection-type LCD. In the present specification, the shortest distance Pcc between peak points of corner cubes will be referred to as the “pitch” of corner cubes. For example, the pitch Pcc of an MCCA is equal to or greater than the wavelength of visible light, and equal to or less than the width of each pixel in a reflection-type LCD.
In the display device as shown in FIGS. 1A and 1B, as described above, display is performed by applying a voltage across the liquid crystal layer 113 so as to change its optical characteristics. For example, in the case where a liquid crystal layer 113 of polymer dispersed liquid crystal (PDLC) is employed, the difference in refractive index between the polymer and the liquid crystal changes in accordance with a voltage applied across the liquid crystal layer 113, so that the degree of scattering in the liquid crystal layer 113 is varied, whereby gray-scale displaying is achieved. Therefore, the thickness of the liquid crystal layer 113 is an important parameter in controlling the displaying by such a display device.
The thickness of the liquid crystal layer 113 is defined by spacers which are provided between the rear substrate 109 (on which the retroreflective layer 106 is provided) and the front substrate 110. Hereinafter, a method which is commonly used for forming spacers in an LCD will be described.
As a method for forming spacers, a method of scattering spacers of a predetermined particle size on either one of the front and rear substrates (spacer scattering method) has conventionally been employed. However, with a spacer scattering method, spacers are randomly disposed on the substrate surface, and it is impossible to dispose spacers in any specific positions on the substrate surface. Therefore, random disorientations in liquid crystal may be caused by the spacers, resulting in improper display.
Therefore, instead of a spacer scattering method, a method which uses photolithography to form columnar spacers on either one of the front and rear substrates is being practiced. According to this method, spacers can be disposed in desired positions on the substrate. Thus, unlike in a spacer scattering method, improper display associated with spacers can be reduced.
Moreover, Japanese Laid-Open Patent Publication No. 2002-055359 discloses a method of forming spacers on a transmission-type LCD, in which columnar spacers are formed by pressing a mold having a surface configuration which defines spacers and contact holes against a resin layer that is provided on the rear substrate. According to this method, spacers can be disposed in desired positions, without performing a photoprocess.
When applying the aforementioned conventional spacer forming methods to a retroreflection-type display device in which an MCCA is employed as the retroreflective layer 106, the following problems may arise.
When a spacer scattering method is employed, spacers are randomly disposed on the retroreflective layer 106, thus resulting in a problem in that sufficient retroreflection characteristics are not exhibited in any regions of the retroreflective layer 106 where spacers are provided (hereinafter “spacer-forming regions”). As a result, the retroreflectance of the retroreflective layer 106 is lowered.
In a spacer forming method which utilizes photolithography, a photoprocess is required for forming spacers, thus resulting in an increase in tact time and cost. Moreover, in order to uniformize the thickness of the liquid crystal layer 113 by reducing the influence of the protrusions and depressions of the retroreflective layer 106, it is desirable to prescribe the diameter (in the case of cylindrical spacers, for example) of the bottom faces of the spacers so as to be greater than the pitch of the retroreflective layer 106, which results in a problem of a lowered aperture ratio.
The aforementioned problem will be specifically described with reference to the figures. FIG. 3A is a schematic cross-sectional view illustrating an exemplary structure of a reflection-type LCD that is produced by using a spacer forming method which utilizes photolithography. The reflection-type LCD shown in FIG. 3A has a similar structure to that of the reflection-type LCD shown in FIGS. 1A and 1B (like reference numerals are given to like constituent elements). FIG. 3B is an upper plan view illustrating an exemplary relationship between the retroreflective layer 106 and a region in which a spacer 115 is formed (spacer-forming region 115r) in the reflection-type LCD shown in FIG. 3A. In the illustrated example, the spacers 115 are quadrangular prisms, and thus the spacer-forming region 115r is represented as a rectangular shape.
In a method which utilizes photolithography, it is possible to control the positions of the spacer-forming regions 115r on the substrate, but it is difficult to precisely align the spacer-forming regions 115r with the minute protrusions and depressions of the retroreflective layer 106. Therefore, as can be seen from FIGS. 3A and 3B, in order to prevent changes in thickness of the liquid crystal layer 113 due to the protrusions and depressions of the retroreflective layer 106, it is necessary to prescribe the width of each spacer-forming region 115r so as to be greater than the pitch Pcc of the corner cubes of the retroreflective layer 106. For example, the width of each spacer-forming region 115r is prescribed to be twice the pitch Pcc or greater, so that the bottom faces of the spacers 115 will be supported more firmly by a plurality of peak points of the retroreflective layer 106. However, as the spacers 115 increase in size, there will be an increasing proportion occupied by the spacer-forming regions 115r in the retroreflective layer 106, thus deteriorating the retroreflection characteristics of the retroreflective layer 106 and lowering the aperture ratio of the display device.
Although Japanese Laid-Open Patent Publication No. 2002-055359 discloses a transfer-based method of forming spacers in a transmission-type display device, it lacks any description as to applying this method to a reflection-type display device which comprises a reflective layer between the rear substrate and the liquid crystal layer. The inventors have conducted a study to find that, when applying this method to a retroreflection-type display device as shown in FIGS. 1A and 1B, one possible technique might be to form a resin layer on the rear substrate 109 (on which the retroreflective layer 106 has been formed), and then transfer a predetermined surface configuration onto the resin layer to form the spacers, for example. According to this technique, the surface of the retroreflective layer 106 will be planarized by the resin layer, thus making it unnecessary to prescribe a large size for the spacers as in the aforementioned photolithography-based method, so that decrease in the aperture ratio can be suppressed. However, there is a problem in that the spacers will be formed on the viewer side of the retroreflective layer 106, as in the other methods, thus resulting in a deteriorated retroreflectance of the retroreflective layer 106. Moreover, this technique also has problems in that the production process will be complicated due to the additional transfer step, and that the thickness of the entire display device will increase because of the resin layer formed between the retroreflective layer 106 and the liquid crystal layer 113.
Moreover, in the reflection-type display device shown in FIGS. 1A and 1B, the reflective metal layer 105 of the retroreflective layer 106 and the switching elements are connected via contact holes 104. However, any region of the retroreflective layer 106 where a contact hole 104 is formed (hereinafter “contact hole-forming region”) may also contribute to deterioration in the retroreflectance of the retroreflective layer 106, as is the case with the spacer-forming regions.
Thus, conventional retroreflection-type display devices have problems in that sufficient retroreflection characteristics cannot be exhibited in the spacer-forming regions and contact hole-forming regions of the retroreflective layer, thus resulting in deterioration of the retroreflectance of the retroreflective layer.